1. FIELD OF THE INVENTION
This invention relates to power supplies and, more specifically, to a power supply composed of plural power supplies, connected in parallel, wherein each power supply may have a different output voltage caused by tolerances between supplies of the same type.
2. BRIEF DESCRIPTION OF THE PRIOR ART
Solid state radar systems and systems using very high speed integrated circuit (VHSIC) technology require power supplies which deliver low voltages and very high currents. An effective method of supplying such high currents is to operate several power supplies in parallel. In such systems, it is desirable for the parallel connected power supplies to share current equally in order to optimize feedback loop response and system reliability.
In general, any two power supplies of the same type, for example supply "A" and supply "B", as shown in FIG. 1 will have different output voltages which, without current sharing provisions, will cause a large imbalance of output current. The supply with the higher output voltage, for example supply "B", will source all the load current until it reaches current limit. After supply "B" goes into current limit and the output voltage drops to the regulation point of supply "A", supply "A" will start to supply current and thereby add some current to the load, sufficient to maintain the output voltage equal to the regulation voltage of supply "A". It can therefore be seen that connecting of plural power supplies in parallel, without other controls, results in the power supply of higher output voltage supplying most of the current to the system until current limit of that supply is reached and the output voltage drops to that of the power supply with next lower output voltage, whereupon the next lower output voltage power supply will commence supply of current to the system. This procedure continues in this manner when additional parallel connected power supplies are present. It is readily apparent that such operation will generally cause breakdown of the highest output voltage power supply prior to that of the other power supplies of the system.
The above described problem is known and several prior art procedures for solution thereof are known. One known solution to the problem of non-balanced output current is introduction of output voltage slope to balance the load current of parallel connected power supplies, which allows the output voltages of the power supplies to match without requiring one of the supplies to be in current limit. The industry has endeavored to provide such voltage slope using minimum power and/or circuitry and circuits therefor have been designed in the past.
In power supply systems wherein the individual supplies are not identical, the output current of the individual power supply with the higher output voltage will be higher than the individual power supply with the lower voltage by a fixed value delta I. For example, if the power supplies are 5 volt supplies with 1% total difference between output voltages and a delta I of 10 amps is desired, a slope of 5 mV/A will be required. Over an output current range from 0 amps to 40 amps, a slope of 5 mV/A corresponds to a load regulation of .+-.2%.
One prior art system for allowing the output voltage to drop off with increasing output current to provide such slope involves the placement of a resistor in series with the output of each of the individual power supplies. A problem with this procedure is that it is very dissipative due to the I.sup.2 R losses developed in the resistor at the output. For example, if the output resistor is 5 milliohms and the output current is 40 amperes, the dissipated power is 8 watts. It is therefore readily apparent that a less dissipative system for providing the desired output voltage slope with increase in output current is required.
Ultimately, power supply voltage regulation is controlled by the error voltage, this being the output of the error amplifier. Feeding the voltage analog of the peak output current into the inverting input of the error amplifier will cause output voltage Vo of the system to decrease with increasing output current. The voltage analog of the power supply output current is a signal used to limit supply current and, as such, is already present in the power supply. Prior art derivation of the peak value of the voltage analog of the power supply output current requires peak detection circuitry which is unique to the programmed output slope. Peak detectors that are highly accurate and wide bandwidth are complicated, using many components. Reducing peak detector parts count sacrifices accuracy, bandwidth, or both. Traditionally, peak detectors have been added to power supply control circuits and the parts count-performance trade-off has been a problem.